Is it possible to terminate a running thread without setting/checking any flags/semaphores/etc.?
This question is related to
python
multithreading
kill
terminate
While it's rather old, this might be a handy solution for some:
A little module that extends the threading's module functionality -- allows one thread to raise exceptions in the context of another thread. By raising
SystemExit, you can finally kill python threads.
import threading
import ctypes
def _async_raise(tid, excobj):
res = ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, ctypes.py_object(excobj))
if res == 0:
raise ValueError("nonexistent thread id")
elif res > 1:
# """if it returns a number greater than one, you're in trouble,
# and you should call it again with exc=NULL to revert the effect"""
ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, 0)
raise SystemError("PyThreadState_SetAsyncExc failed")
class Thread(threading.Thread):
def raise_exc(self, excobj):
assert self.isAlive(), "thread must be started"
for tid, tobj in threading._active.items():
if tobj is self:
_async_raise(tid, excobj)
return
# the thread was alive when we entered the loop, but was not found
# in the dict, hence it must have been already terminated. should we raise
# an exception here? silently ignore?
def terminate(self):
# must raise the SystemExit type, instead of a SystemExit() instance
# due to a bug in PyThreadState_SetAsyncExc
self.raise_exc(SystemExit)
So, it allows a "thread to raise exceptions in the context of another thread" and in this way, the terminated thread can handle the termination without regularly checking an abort flag.
However, according to its original source, there are some issues with this code.
- The exception will be raised only when executing python bytecode. If your thread calls a native/built-in blocking function, the exception will be raised only when execution returns to the python code.
- There is also an issue if the built-in function internally calls PyErr_Clear(), which would effectively cancel your pending exception. You can try to raise it again.
- Only exception types can be raised safely. Exception instances are likely to cause unexpected behavior, and are thus restricted.
- For example: t1.raise_exc(TypeError) and not t1.raise_exc(TypeError("blah")).
- IMHO it's a bug, and I reported it as one. For more info, http://mail.python.org/pipermail/python-dev/2006-August/068158.html
- I asked to expose this function in the built-in thread module, but since ctypes has become a standard library (as of 2.5), and this
feature is not likely to be implementation-agnostic, it may be kept
unexposed.
In Python, you simply cannot kill a Thread directly.
If you do NOT really need to have a Thread (!), what you can do, instead of using the threading package , is to use the multiprocessing package . Here, to kill a process, you can simply call the method:
yourProcess.terminate() # kill the process!
Python will kill your process (on Unix through the SIGTERM signal, while on Windows through the TerminateProcess() call). Pay attention to use it while using a Queue or a Pipe! (it may corrupt the data in the Queue/Pipe)
Note that the multiprocessing.Event and the multiprocessing.Semaphore work exactly in the same way of the threading.Event and the threading.Semaphore respectively. In fact, the first ones are clones of the latters.
If you REALLY need to use a Thread, there is no way to kill it directly. What you can do, however, is to use a "daemon thread". In fact, in Python, a Thread can be flagged as daemon:
yourThread.daemon = True # set the Thread as a "daemon thread"
The main program will exit when no alive non-daemon threads are left. In other words, when your main thread (which is, of course, a non-daemon thread) will finish its operations, the program will exit even if there are still some daemon threads working.
Note that it is necessary to set a Thread as daemon before the start() method is called!
Of course you can, and should, use daemon even with multiprocessing. Here, when the main process exits, it attempts to terminate all of its daemonic child processes.
Finally, please, note that sys.exit() and os.kill() are not choices.
This seems to work with pywin32 on windows 7
my_thread = threading.Thread()
my_thread.start()
my_thread._Thread__stop()
In Python, you simply cannot kill a Thread directly.
If you do NOT really need to have a Thread (!), what you can do, instead of using the threading package , is to use the multiprocessing package . Here, to kill a process, you can simply call the method:
yourProcess.terminate() # kill the process!
Python will kill your process (on Unix through the SIGTERM signal, while on Windows through the TerminateProcess() call). Pay attention to use it while using a Queue or a Pipe! (it may corrupt the data in the Queue/Pipe)
Note that the multiprocessing.Event and the multiprocessing.Semaphore work exactly in the same way of the threading.Event and the threading.Semaphore respectively. In fact, the first ones are clones of the latters.
If you REALLY need to use a Thread, there is no way to kill it directly. What you can do, however, is to use a "daemon thread". In fact, in Python, a Thread can be flagged as daemon:
yourThread.daemon = True # set the Thread as a "daemon thread"
The main program will exit when no alive non-daemon threads are left. In other words, when your main thread (which is, of course, a non-daemon thread) will finish its operations, the program will exit even if there are still some daemon threads working.
Note that it is necessary to set a Thread as daemon before the start() method is called!
Of course you can, and should, use daemon even with multiprocessing. Here, when the main process exits, it attempts to terminate all of its daemonic child processes.
Finally, please, note that sys.exit() and os.kill() are not choices.
As mentioned in @Kozyarchuk's answer, installing trace works. Since this answer contained no code, here is a working ready-to-use example:
import sys, threading, time
class TraceThread(threading.Thread):
def __init__(self, *args, **keywords):
threading.Thread.__init__(self, *args, **keywords)
self.killed = False
def start(self):
self._run = self.run
self.run = self.settrace_and_run
threading.Thread.start(self)
def settrace_and_run(self):
sys.settrace(self.globaltrace)
self._run()
def globaltrace(self, frame, event, arg):
return self.localtrace if event == 'call' else None
def localtrace(self, frame, event, arg):
if self.killed and event == 'line':
raise SystemExit()
return self.localtrace
def f():
while True:
print('1')
time.sleep(2)
print('2')
time.sleep(2)
print('3')
time.sleep(2)
t = TraceThread(target=f)
t.start()
time.sleep(2.5)
t.killed = True
It stops after having printed 1 and 2. 3 is not printed.
Start the sub thread with setDaemon(True).
def bootstrap(_filename):
mb = ModelBootstrap(filename=_filename) # Has many Daemon threads. All get stopped automatically when main thread is stopped.
t = threading.Thread(target=bootstrap,args=('models.conf',))
t.setDaemon(False)
while True:
t.start()
time.sleep(10) # I am just allowing the sub-thread to run for 10 sec. You can listen on an event to stop execution.
print('Thread stopped')
break
from ctypes import *
pthread = cdll.LoadLibrary("libpthread-2.15.so")
pthread.pthread_cancel(c_ulong(t.ident))
t is your Thread object.
Read the python source (Modules/threadmodule.c and Python/thread_pthread.h) you can see the Thread.ident is an pthread_t type, so you can do anything pthread can do in python use libpthread.
This is based on thread2 -- killable threads (Python recipe)
You need to call PyThreadState_SetasyncExc(), which is only available through ctypes.
This has only been tested on Python 2.7.3, but it is likely to work with other recent 2.x releases.
import ctypes
def terminate_thread(thread):
"""Terminates a python thread from another thread.
:param thread: a threading.Thread instance
"""
if not thread.isAlive():
return
exc = ctypes.py_object(SystemExit)
res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
ctypes.c_long(thread.ident), exc)
if res == 0:
raise ValueError("nonexistent thread id")
elif res > 1:
# """if it returns a number greater than one, you're in trouble,
# and you should call it again with exc=NULL to revert the effect"""
ctypes.pythonapi.PyThreadState_SetAsyncExc(thread.ident, None)
raise SystemError("PyThreadState_SetAsyncExc failed")
from ctypes import *
pthread = cdll.LoadLibrary("libpthread-2.15.so")
pthread.pthread_cancel(c_ulong(t.ident))
t is your Thread object.
Read the python source (Modules/threadmodule.c and Python/thread_pthread.h) you can see the Thread.ident is an pthread_t type, so you can do anything pthread can do in python use libpthread.
Here's yet another way to do it, but with extremely clean and simple code, that works in Python 3.7 in 2021:
import ctypes
def kill_thread(thread):
"""
thread: a threading.Thread object
"""
thread_id = thread.ident
res = ctypes.pythonapi.PyThreadState_SetAsyncExc(thread_id, ctypes.py_object(SystemExit))
if res > 1:
ctypes.pythonapi.PyThreadState_SetAsyncExc(thread_id, 0)
print('Exception raise failure')
Adapted from here: https://www.geeksforgeeks.org/python-different-ways-to-kill-a-thread/
If you are explicitly calling time.sleep() as part of your thread (say polling some external service), an improvement upon Phillipe's method is to use the timeout in the event's wait() method wherever you sleep()
For example:
import threading
class KillableThread(threading.Thread):
def __init__(self, sleep_interval=1):
super().__init__()
self._kill = threading.Event()
self._interval = sleep_interval
def run(self):
while True:
print("Do Something")
# If no kill signal is set, sleep for the interval,
# If kill signal comes in while sleeping, immediately
# wake up and handle
is_killed = self._kill.wait(self._interval)
if is_killed:
break
print("Killing Thread")
def kill(self):
self._kill.set()
Then to run it
t = KillableThread(sleep_interval=5)
t.start()
# Every 5 seconds it prints:
#: Do Something
t.kill()
#: Killing Thread
The advantage of using wait() instead of sleep()ing and regularly checking the event is that you can program in longer intervals of sleep, the thread is stopped almost immediately (when you would otherwise be sleep()ing) and in my opinion, the code for handling exit is significantly simpler.
You can kill a thread by installing trace into the thread that will exit the thread. See attached link for one possible implementation.
Start the sub thread with setDaemon(True).
def bootstrap(_filename):
mb = ModelBootstrap(filename=_filename) # Has many Daemon threads. All get stopped automatically when main thread is stopped.
t = threading.Thread(target=bootstrap,args=('models.conf',))
t.setDaemon(False)
while True:
t.start()
time.sleep(10) # I am just allowing the sub-thread to run for 10 sec. You can listen on an event to stop execution.
print('Thread stopped')
break
As mentioned in @Kozyarchuk's answer, installing trace works. Since this answer contained no code, here is a working ready-to-use example:
import sys, threading, time
class TraceThread(threading.Thread):
def __init__(self, *args, **keywords):
threading.Thread.__init__(self, *args, **keywords)
self.killed = False
def start(self):
self._run = self.run
self.run = self.settrace_and_run
threading.Thread.start(self)
def settrace_and_run(self):
sys.settrace(self.globaltrace)
self._run()
def globaltrace(self, frame, event, arg):
return self.localtrace if event == 'call' else None
def localtrace(self, frame, event, arg):
if self.killed and event == 'line':
raise SystemExit()
return self.localtrace
def f():
while True:
print('1')
time.sleep(2)
print('2')
time.sleep(2)
print('3')
time.sleep(2)
t = TraceThread(target=f)
t.start()
time.sleep(2.5)
t.killed = True
It stops after having printed 1 and 2. 3 is not printed.
You can execute your command in a process and then kill it using the process id. I needed to sync between two thread one of which doesn’t return by itself.
processIds = []
def executeRecord(command):
print(command)
process = subprocess.Popen(command, stdout=subprocess.PIPE)
processIds.append(process.pid)
print(processIds[0])
#Command that doesn't return by itself
process.stdout.read().decode("utf-8")
return;
def recordThread(command, timeOut):
thread = Thread(target=executeRecord, args=(command,))
thread.start()
thread.join(timeOut)
os.kill(processIds.pop(), signal.SIGINT)
return;
This is a bad answer, see the comments
Here's how to do it:
from threading import *
...
for thread in enumerate():
if thread.isAlive():
try:
thread._Thread__stop()
except:
print(str(thread.getName()) + ' could not be terminated'))
Give it a few seconds then your thread should be stopped. Check also the thread._Thread__delete() method.
I'd recommend a thread.quit() method for convenience. For example if you have a socket in your thread, I'd recommend creating a quit() method in your socket-handle class, terminate the socket, then run a thread._Thread__stop() inside of your quit().
It is better if you don't kill a thread. A way could be to introduce a "try" block into the thread's cycle and to throw an exception when you want to stop the thread (for example a break/return/... that stops your for/while/...). I've used this on my app and it works...
This is based on thread2 -- killable threads (Python recipe)
You need to call PyThreadState_SetasyncExc(), which is only available through ctypes.
This has only been tested on Python 2.7.3, but it is likely to work with other recent 2.x releases.
import ctypes
def terminate_thread(thread):
"""Terminates a python thread from another thread.
:param thread: a threading.Thread instance
"""
if not thread.isAlive():
return
exc = ctypes.py_object(SystemExit)
res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
ctypes.c_long(thread.ident), exc)
if res == 0:
raise ValueError("nonexistent thread id")
elif res > 1:
# """if it returns a number greater than one, you're in trouble,
# and you should call it again with exc=NULL to revert the effect"""
ctypes.pythonapi.PyThreadState_SetAsyncExc(thread.ident, None)
raise SystemError("PyThreadState_SetAsyncExc failed")
There is a library built for this purpose, stopit. Although some of the same cautions listed herein still apply, at least this library presents a regular, repeatable technique for achieving the stated goal.
You can kill a thread by installing trace into the thread that will exit the thread. See attached link for one possible implementation.
Asuming, that you want to have multiple threads of the same function, this is IMHO the easiest implementation to stop one by id:
import time
from threading import Thread
def doit(id=0):
doit.stop=0
print("start id:%d"%id)
while 1:
time.sleep(1)
print(".")
if doit.stop==id:
doit.stop=0
break
print("end thread %d"%id)
t5=Thread(target=doit, args=(5,))
t6=Thread(target=doit, args=(6,))
t5.start() ; t6.start()
time.sleep(2)
doit.stop =5 #kill t5
time.sleep(2)
doit.stop =6 #kill t6
The nice thing is here, you can have multiple of same and different functions, and stop them all by functionname.stop
If you want to have only one thread of the function then you don't need to remember the id. Just stop, if doit.stop > 0.
Pieter Hintjens -- one of the founders of the ØMQ-project -- says, using ØMQ and avoiding synchronization primitives like locks, mutexes, events etc., is the sanest and securest way to write multi-threaded programs:
http://zguide.zeromq.org/py:all#Multithreading-with-ZeroMQ
This includes telling a child thread, that it should cancel its work. This would be done by equipping the thread with a ØMQ-socket and polling on that socket for a message saying that it should cancel.
The link also provides an example on multi-threaded python code with ØMQ.
Following workaround can be used to kill a thread:
kill_threads = False
def doSomething():
global kill_threads
while True:
if kill_threads:
thread.exit()
......
......
thread.start_new_thread(doSomething, ())
This can be used even for terminating threads, whose code is written in another module, from main thread. We can declare a global variable in that module and use it to terminate thread/s spawned in that module.
I usually use this to terminate all the threads at the program exit. This might not be the perfect way to terminate thread/s but could help.
Just to build up on @SCB's idea (which was exactly what I needed) to create a KillableThread subclass with a customized function:
from threading import Thread, Event
class KillableThread(Thread):
def __init__(self, sleep_interval=1, target=None, name=None, args=(), kwargs={}):
super().__init__(None, target, name, args, kwargs)
self._kill = Event()
self._interval = sleep_interval
print(self._target)
def run(self):
while True:
# Call custom function with arguments
self._target(*self._args)
# If no kill signal is set, sleep for the interval,
# If kill signal comes in while sleeping, immediately
# wake up and handle
is_killed = self._kill.wait(self._interval)
if is_killed:
break
print("Killing Thread")
def kill(self):
self._kill.set()
if __name__ == '__main__':
def print_msg(msg):
print(msg)
t = KillableThread(10, print_msg, args=("hello world"))
t.start()
time.sleep(6)
print("About to kill thread")
t.kill()
Naturally, like with @SBC, the thread doesn't wait to run a new loop to stop. In this example, you would see the "Killing Thread" message printed right after the "About to kill thread" instead of waiting for 4 more seconds for the thread to complete (since we have slept for 6 seconds already).
Second argument in KillableThread constructor is your custom function (print_msg here). Args argument are the arguments that will be used when calling the function (("hello world")) here.
There is no official API to do that, no.
You need to use platform API to kill the thread, e.g. pthread_kill, or TerminateThread. You can access such API e.g. through pythonwin, or through ctypes.
Notice that this is inherently unsafe. It will likely lead to uncollectable garbage (from local variables of the stack frames that become garbage), and may lead to deadlocks, if the thread being killed has the GIL at the point when it is killed.
You can execute your command in a process and then kill it using the process id. I needed to sync between two thread one of which doesn’t return by itself.
processIds = []
def executeRecord(command):
print(command)
process = subprocess.Popen(command, stdout=subprocess.PIPE)
processIds.append(process.pid)
print(processIds[0])
#Command that doesn't return by itself
process.stdout.read().decode("utf-8")
return;
def recordThread(command, timeOut):
thread = Thread(target=executeRecord, args=(command,))
thread.start()
thread.join(timeOut)
os.kill(processIds.pop(), signal.SIGINT)
return;
Here's yet another way to do it, but with extremely clean and simple code, that works in Python 3.7 in 2021:
import ctypes
def kill_thread(thread):
"""
thread: a threading.Thread object
"""
thread_id = thread.ident
res = ctypes.pythonapi.PyThreadState_SetAsyncExc(thread_id, ctypes.py_object(SystemExit))
if res > 1:
ctypes.pythonapi.PyThreadState_SetAsyncExc(thread_id, 0)
print('Exception raise failure')
Adapted from here: https://www.geeksforgeeks.org/python-different-ways-to-kill-a-thread/
While it's rather old, this might be a handy solution for some:
A little module that extends the threading's module functionality -- allows one thread to raise exceptions in the context of another thread. By raising
SystemExit, you can finally kill python threads.
import threading
import ctypes
def _async_raise(tid, excobj):
res = ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, ctypes.py_object(excobj))
if res == 0:
raise ValueError("nonexistent thread id")
elif res > 1:
# """if it returns a number greater than one, you're in trouble,
# and you should call it again with exc=NULL to revert the effect"""
ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, 0)
raise SystemError("PyThreadState_SetAsyncExc failed")
class Thread(threading.Thread):
def raise_exc(self, excobj):
assert self.isAlive(), "thread must be started"
for tid, tobj in threading._active.items():
if tobj is self:
_async_raise(tid, excobj)
return
# the thread was alive when we entered the loop, but was not found
# in the dict, hence it must have been already terminated. should we raise
# an exception here? silently ignore?
def terminate(self):
# must raise the SystemExit type, instead of a SystemExit() instance
# due to a bug in PyThreadState_SetAsyncExc
self.raise_exc(SystemExit)
So, it allows a "thread to raise exceptions in the context of another thread" and in this way, the terminated thread can handle the termination without regularly checking an abort flag.
However, according to its original source, there are some issues with this code.
- The exception will be raised only when executing python bytecode. If your thread calls a native/built-in blocking function, the exception will be raised only when execution returns to the python code.
- There is also an issue if the built-in function internally calls PyErr_Clear(), which would effectively cancel your pending exception. You can try to raise it again.
- Only exception types can be raised safely. Exception instances are likely to cause unexpected behavior, and are thus restricted.
- For example: t1.raise_exc(TypeError) and not t1.raise_exc(TypeError("blah")).
- IMHO it's a bug, and I reported it as one. For more info, http://mail.python.org/pipermail/python-dev/2006-August/068158.html
- I asked to expose this function in the built-in thread module, but since ctypes has become a standard library (as of 2.5), and this
feature is not likely to be implementation-agnostic, it may be kept
unexposed.
There is a library built for this purpose, stopit. Although some of the same cautions listed herein still apply, at least this library presents a regular, repeatable technique for achieving the stated goal.
If you are trying to terminate the whole program you can set the thread as a "daemon". see Thread.daemon
Following workaround can be used to kill a thread:
kill_threads = False
def doSomething():
global kill_threads
while True:
if kill_threads:
thread.exit()
......
......
thread.start_new_thread(doSomething, ())
This can be used even for terminating threads, whose code is written in another module, from main thread. We can declare a global variable in that module and use it to terminate thread/s spawned in that module.
I usually use this to terminate all the threads at the program exit. This might not be the perfect way to terminate thread/s but could help.
If you are trying to terminate the whole program you can set the thread as a "daemon". see Thread.daemon
You should never forcibly kill a thread without cooperating with it.
Killing a thread removes any guarantees that try/finally blocks set up so you might leave locks locked, files open, etc.
The only time you can argue that forcibly killing threads is a good idea is to kill a program fast, but never single threads.
As others have mentioned, the norm is to set a stop flag. For something lightweight (no subclassing of Thread, no global variable), a lambda callback is an option. (Note the parentheses in if stop().)
import threading
import time
def do_work(id, stop):
print("I am thread", id)
while True:
print("I am thread {} doing something".format(id))
if stop():
print(" Exiting loop.")
break
print("Thread {}, signing off".format(id))
def main():
stop_threads = False
workers = []
for id in range(0,3):
tmp = threading.Thread(target=do_work, args=(id, lambda: stop_threads))
workers.append(tmp)
tmp.start()
time.sleep(3)
print('main: done sleeping; time to stop the threads.')
stop_threads = True
for worker in workers:
worker.join()
print('Finis.')
if __name__ == '__main__':
main()
Replacing print() with a pr() function that always flushes (sys.stdout.flush()) may improve the precision of the shell output.
(Only tested on Windows/Eclipse/Python3.3)
Asuming, that you want to have multiple threads of the same function, this is IMHO the easiest implementation to stop one by id:
import time
from threading import Thread
def doit(id=0):
doit.stop=0
print("start id:%d"%id)
while 1:
time.sleep(1)
print(".")
if doit.stop==id:
doit.stop=0
break
print("end thread %d"%id)
t5=Thread(target=doit, args=(5,))
t6=Thread(target=doit, args=(6,))
t5.start() ; t6.start()
time.sleep(2)
doit.stop =5 #kill t5
time.sleep(2)
doit.stop =6 #kill t6
The nice thing is here, you can have multiple of same and different functions, and stop them all by functionname.stop
If you want to have only one thread of the function then you don't need to remember the id. Just stop, if doit.stop > 0.
You can kill a thread by installing trace into the thread that will exit the thread. See attached link for one possible implementation.
You should never forcibly kill a thread without cooperating with it.
Killing a thread removes any guarantees that try/finally blocks set up so you might leave locks locked, files open, etc.
The only time you can argue that forcibly killing threads is a good idea is to kill a program fast, but never single threads.
It is definitely possible to implement a Thread.stop method as shown in the following example code:
import sys
import threading
import time
class StopThread(StopIteration):
pass
threading.SystemExit = SystemExit, StopThread
class Thread2(threading.Thread):
def stop(self):
self.__stop = True
def _bootstrap(self):
if threading._trace_hook is not None:
raise ValueError('Cannot run thread with tracing!')
self.__stop = False
sys.settrace(self.__trace)
super()._bootstrap()
def __trace(self, frame, event, arg):
if self.__stop:
raise StopThread()
return self.__trace
class Thread3(threading.Thread):
def _bootstrap(self, stop_thread=False):
def stop():
nonlocal stop_thread
stop_thread = True
self.stop = stop
def tracer(*_):
if stop_thread:
raise StopThread()
return tracer
sys.settrace(tracer)
super()._bootstrap()
###############################################################################
def main():
test1 = Thread2(target=printer)
test1.start()
time.sleep(1)
test1.stop()
test1.join()
test2 = Thread2(target=speed_test)
test2.start()
time.sleep(1)
test2.stop()
test2.join()
test3 = Thread3(target=speed_test)
test3.start()
time.sleep(1)
test3.stop()
test3.join()
def printer():
while True:
print(time.time() % 1)
time.sleep(0.1)
def speed_test(count=0):
try:
while True:
count += 1
except StopThread:
print('Count =', count)
if __name__ == '__main__':
main()
The Thread3 class appears to run code approximately 33% faster than the Thread2 class.
I'm way late to this game, but I've been wrestling with a similar question and the following appears to both resolve the issue perfectly for me AND lets me do some basic thread state checking and cleanup when the daemonized sub-thread exits:
import threading
import time
import atexit
def do_work():
i = 0
@atexit.register
def goodbye():
print ("'CLEANLY' kill sub-thread with value: %s [THREAD: %s]" %
(i, threading.currentThread().ident))
while True:
print i
i += 1
time.sleep(1)
t = threading.Thread(target=do_work)
t.daemon = True
t.start()
def after_timeout():
print "KILL MAIN THREAD: %s" % threading.currentThread().ident
raise SystemExit
threading.Timer(2, after_timeout).start()
Yields:
0
1
KILL MAIN THREAD: 140013208254208
'CLEANLY' kill sub-thread with value: 2 [THREAD: 140013674317568]
A multiprocessing.Process can p.terminate()
In the cases where I want to kill a thread, but do not want to use flags/locks/signals/semaphores/events/whatever, I promote the threads to full blown processes. For code that makes use of just a few threads the overhead is not that bad.
E.g. this comes in handy to easily terminate helper "threads" which execute blocking I/O
The conversion is trivial: In related code replace all threading.Thread with multiprocessing.Process and all queue.Queue with multiprocessing.Queue and add the required calls of p.terminate() to your parent process which wants to kill its child p
See the Python documentation for multiprocessing.
Example:
import multiprocessing
proc = multiprocessing.Process(target=your_proc_function, args=())
proc.start()
# Terminate the process
proc.terminate() # sends a SIGTERM
This is a bad answer, see the comments
Here's how to do it:
from threading import *
...
for thread in enumerate():
if thread.isAlive():
try:
thread._Thread__stop()
except:
print(str(thread.getName()) + ' could not be terminated'))
Give it a few seconds then your thread should be stopped. Check also the thread._Thread__delete() method.
I'd recommend a thread.quit() method for convenience. For example if you have a socket in your thread, I'd recommend creating a quit() method in your socket-handle class, terminate the socket, then run a thread._Thread__stop() inside of your quit().
One thing I want to add is that if you read official documentation in threading lib Python, it's recommended to avoid use of "demonic" threads, when you don't want threads end abruptly, with the flag that Paolo Rovelli mentioned.
From official documentation:
Daemon threads are abruptly stopped at shutdown. Their resources (such as open files, database transactions, etc.) may not be released properly. If you want your threads to stop gracefully, make them non-daemonic and use a suitable signaling mechanism such as an Event.
I think that creating daemonic threads depends of your application, but in general (and in my opinion) it's better to avoid killing them or making them daemonic. In multiprocessing you can use is_alive() to check process status and "terminate" for finish them (Also you avoid GIL problems). But you can find more problems, sometimes, when you execute your code in Windows.
And always remember that if you have "live threads", the Python interpreter will be running for wait them. (Because of this daemonic can help you if don't matter abruptly ends).
Just to build up on @SCB's idea (which was exactly what I needed) to create a KillableThread subclass with a customized function:
from threading import Thread, Event
class KillableThread(Thread):
def __init__(self, sleep_interval=1, target=None, name=None, args=(), kwargs={}):
super().__init__(None, target, name, args, kwargs)
self._kill = Event()
self._interval = sleep_interval
print(self._target)
def run(self):
while True:
# Call custom function with arguments
self._target(*self._args)
# If no kill signal is set, sleep for the interval,
# If kill signal comes in while sleeping, immediately
# wake up and handle
is_killed = self._kill.wait(self._interval)
if is_killed:
break
print("Killing Thread")
def kill(self):
self._kill.set()
if __name__ == '__main__':
def print_msg(msg):
print(msg)
t = KillableThread(10, print_msg, args=("hello world"))
t.start()
time.sleep(6)
print("About to kill thread")
t.kill()
Naturally, like with @SBC, the thread doesn't wait to run a new loop to stop. In this example, you would see the "Killing Thread" message printed right after the "About to kill thread" instead of waiting for 4 more seconds for the thread to complete (since we have slept for 6 seconds already).
Second argument in KillableThread constructor is your custom function (print_msg here). Args argument are the arguments that will be used when calling the function (("hello world")) here.
As others have mentioned, the norm is to set a stop flag. For something lightweight (no subclassing of Thread, no global variable), a lambda callback is an option. (Note the parentheses in if stop().)
import threading
import time
def do_work(id, stop):
print("I am thread", id)
while True:
print("I am thread {} doing something".format(id))
if stop():
print(" Exiting loop.")
break
print("Thread {}, signing off".format(id))
def main():
stop_threads = False
workers = []
for id in range(0,3):
tmp = threading.Thread(target=do_work, args=(id, lambda: stop_threads))
workers.append(tmp)
tmp.start()
time.sleep(3)
print('main: done sleeping; time to stop the threads.')
stop_threads = True
for worker in workers:
worker.join()
print('Finis.')
if __name__ == '__main__':
main()
Replacing print() with a pr() function that always flushes (sys.stdout.flush()) may improve the precision of the shell output.
(Only tested on Windows/Eclipse/Python3.3)
You should never forcibly kill a thread without cooperating with it.
Killing a thread removes any guarantees that try/finally blocks set up so you might leave locks locked, files open, etc.
The only time you can argue that forcibly killing threads is a good idea is to kill a program fast, but never single threads.
There is no official API to do that, no.
You need to use platform API to kill the thread, e.g. pthread_kill, or TerminateThread. You can access such API e.g. through pythonwin, or through ctypes.
Notice that this is inherently unsafe. It will likely lead to uncollectable garbage (from local variables of the stack frames that become garbage), and may lead to deadlocks, if the thread being killed has the GIL at the point when it is killed.
It is better if you don't kill a thread. A way could be to introduce a "try" block into the thread's cycle and to throw an exception when you want to stop the thread (for example a break/return/... that stops your for/while/...). I've used this on my app and it works...
This seems to work with pywin32 on windows 7
my_thread = threading.Thread()
my_thread.start()
my_thread._Thread__stop()
If you are explicitly calling time.sleep() as part of your thread (say polling some external service), an improvement upon Phillipe's method is to use the timeout in the event's wait() method wherever you sleep()
For example:
import threading
class KillableThread(threading.Thread):
def __init__(self, sleep_interval=1):
super().__init__()
self._kill = threading.Event()
self._interval = sleep_interval
def run(self):
while True:
print("Do Something")
# If no kill signal is set, sleep for the interval,
# If kill signal comes in while sleeping, immediately
# wake up and handle
is_killed = self._kill.wait(self._interval)
if is_killed:
break
print("Killing Thread")
def kill(self):
self._kill.set()
Then to run it
t = KillableThread(sleep_interval=5)
t.start()
# Every 5 seconds it prints:
#: Do Something
t.kill()
#: Killing Thread
The advantage of using wait() instead of sleep()ing and regularly checking the event is that you can program in longer intervals of sleep, the thread is stopped almost immediately (when you would otherwise be sleep()ing) and in my opinion, the code for handling exit is significantly simpler.
It is definitely possible to implement a Thread.stop method as shown in the following example code:
import sys
import threading
import time
class StopThread(StopIteration):
pass
threading.SystemExit = SystemExit, StopThread
class Thread2(threading.Thread):
def stop(self):
self.__stop = True
def _bootstrap(self):
if threading._trace_hook is not None:
raise ValueError('Cannot run thread with tracing!')
self.__stop = False
sys.settrace(self.__trace)
super()._bootstrap()
def __trace(self, frame, event, arg):
if self.__stop:
raise StopThread()
return self.__trace
class Thread3(threading.Thread):
def _bootstrap(self, stop_thread=False):
def stop():
nonlocal stop_thread
stop_thread = True
self.stop = stop
def tracer(*_):
if stop_thread:
raise StopThread()
return tracer
sys.settrace(tracer)
super()._bootstrap()
###############################################################################
def main():
test1 = Thread2(target=printer)
test1.start()
time.sleep(1)
test1.stop()
test1.join()
test2 = Thread2(target=speed_test)
test2.start()
time.sleep(1)
test2.stop()
test2.join()
test3 = Thread3(target=speed_test)
test3.start()
time.sleep(1)
test3.stop()
test3.join()
def printer():
while True:
print(time.time() % 1)
time.sleep(0.1)
def speed_test(count=0):
try:
while True:
count += 1
except StopThread:
print('Count =', count)
if __name__ == '__main__':
main()
The Thread3 class appears to run code approximately 33% faster than the Thread2 class.
I'm way late to this game, but I've been wrestling with a similar question and the following appears to both resolve the issue perfectly for me AND lets me do some basic thread state checking and cleanup when the daemonized sub-thread exits:
import threading
import time
import atexit
def do_work():
i = 0
@atexit.register
def goodbye():
print ("'CLEANLY' kill sub-thread with value: %s [THREAD: %s]" %
(i, threading.currentThread().ident))
while True:
print i
i += 1
time.sleep(1)
t = threading.Thread(target=do_work)
t.daemon = True
t.start()
def after_timeout():
print "KILL MAIN THREAD: %s" % threading.currentThread().ident
raise SystemExit
threading.Timer(2, after_timeout).start()
Yields:
0
1
KILL MAIN THREAD: 140013208254208
'CLEANLY' kill sub-thread with value: 2 [THREAD: 140013674317568]
You should never forcibly kill a thread without cooperating with it.
Killing a thread removes any guarantees that try/finally blocks set up so you might leave locks locked, files open, etc.
The only time you can argue that forcibly killing threads is a good idea is to kill a program fast, but never single threads.
It is better if you don't kill a thread. A way could be to introduce a "try" block into the thread's cycle and to throw an exception when you want to stop the thread (for example a break/return/... that stops your for/while/...). I've used this on my app and it works...
A multiprocessing.Process can p.terminate()
In the cases where I want to kill a thread, but do not want to use flags/locks/signals/semaphores/events/whatever, I promote the threads to full blown processes. For code that makes use of just a few threads the overhead is not that bad.
E.g. this comes in handy to easily terminate helper "threads" which execute blocking I/O
The conversion is trivial: In related code replace all threading.Thread with multiprocessing.Process and all queue.Queue with multiprocessing.Queue and add the required calls of p.terminate() to your parent process which wants to kill its child p
See the Python documentation for multiprocessing.
Example:
import multiprocessing
proc = multiprocessing.Process(target=your_proc_function, args=())
proc.start()
# Terminate the process
proc.terminate() # sends a SIGTERM
You can kill a thread by installing trace into the thread that will exit the thread. See attached link for one possible implementation.
If you really need the ability to kill a sub-task, use an alternate implementation. multiprocessing and gevent both support indiscriminately killing a "thread".
Python's threading does not support cancellation. Do not even try. Your code is very likely to deadlock, corrupt or leak memory, or have other unintended "interesting" hard-to-debug effects which happen rarely and nondeterministically.
One thing I want to add is that if you read official documentation in threading lib Python, it's recommended to avoid use of "demonic" threads, when you don't want threads end abruptly, with the flag that Paolo Rovelli mentioned.
From official documentation:
Daemon threads are abruptly stopped at shutdown. Their resources (such as open files, database transactions, etc.) may not be released properly. If you want your threads to stop gracefully, make them non-daemonic and use a suitable signaling mechanism such as an Event.
I think that creating daemonic threads depends of your application, but in general (and in my opinion) it's better to avoid killing them or making them daemonic. In multiprocessing you can use is_alive() to check process status and "terminate" for finish them (Also you avoid GIL problems). But you can find more problems, sometimes, when you execute your code in Windows.
And always remember that if you have "live threads", the Python interpreter will be running for wait them. (Because of this daemonic can help you if don't matter abruptly ends).
It is better if you don't kill a thread. A way could be to introduce a "try" block into the thread's cycle and to throw an exception when you want to stop the thread (for example a break/return/... that stops your for/while/...). I've used this on my app and it works...
There is no official API to do that, no.
You need to use platform API to kill the thread, e.g. pthread_kill, or TerminateThread. You can access such API e.g. through pythonwin, or through ctypes.
Notice that this is inherently unsafe. It will likely lead to uncollectable garbage (from local variables of the stack frames that become garbage), and may lead to deadlocks, if the thread being killed has the GIL at the point when it is killed.
If you really need the ability to kill a sub-task, use an alternate implementation. multiprocessing and gevent both support indiscriminately killing a "thread".
Python's threading does not support cancellation. Do not even try. Your code is very likely to deadlock, corrupt or leak memory, or have other unintended "interesting" hard-to-debug effects which happen rarely and nondeterministically.
Pieter Hintjens -- one of the founders of the ØMQ-project -- says, using ØMQ and avoiding synchronization primitives like locks, mutexes, events etc., is the sanest and securest way to write multi-threaded programs:
http://zguide.zeromq.org/py:all#Multithreading-with-ZeroMQ
This includes telling a child thread, that it should cancel its work. This would be done by equipping the thread with a ØMQ-socket and polling on that socket for a message saying that it should cancel.
The link also provides an example on multi-threaded python code with ØMQ.
Source: Stackoverflow.com